1. Field of the Invention
The present invention relates to a device for exciting a gas, of the surfaguide or surfatron-guide type, in which the gas to be excited is treated with a surface-wave plasma, in particular at atmospheric pressure.
2. Description of the Related Art
In order to excite a gas, it is passed through a dielectric tube and an electric discharge, preferably out of local thermodynamic equilibrium, is sustained in this gas by means of the electric field associated with a surface wave. This wave is excited by a field applicator which is itself supplied with microwave power from a conventional waveguide.
A first type of applicator, known by the name surfatron-guide, includes a hollow structure made of electrically conductive material, including a first part which delimits a concentration zone for the incident waves and is closed off by a moving waveguide plunger forming a short-circuit, and a second part which extends perpendicularly to the first part and in which a tube made of dielectric material, through which the gas to be excited is passed, is mounted coaxially. The second part is provided with an axially movable tuning plunger for matching the impedance of the device.
Another known type of applicator, known by the name surfaguide, also includes a hollow structure which forms a waveguide, is made of an electrically conductive material, is intended to be connected to a microwave generator and is provided with a passage intended to be passed through by a dielectric hollow tube through which the excited gas is passed. The hollow structure has a longitudinal general shape and has a wave concentration zone designed to make the microwave power delivered by the generator become concentrated towards the tube during operation of the device, with a view to producing a plasma in the said gas, preferably a surface-wave plasma.
Surfaguides do not have a tuning piston such as the one which exists in the second part of surfatron guides. It is therefore less expensive than the latter. Furthermore, the length of the plasma created by surfaguides is, for equal power, slightly higher than that of the plasma created by surfatron-guides.
However, under certain operating conditions, surfaguides are less efficient than surfatron-guides, when use is made of discharge tubes of diameter in excess of 20 mm at a frequency of 2.45 GHz.
Furthermore, for high operating powers, radiation losses occur in the environment of surfaguides, which are highly prejudicial to the energy balance of the device and furthermore raise problems of reliability and safety.
In addition, because of their construction, these two types of exciter devices have relatively limited conversion yields for the gases to be treated.
Thus, for example, for C.sub.2 F.sub.6, the maximum gas flow rate which it is possible to destroy to more than 90% is of the order of 500 standard cm.sup.3 per minute (SCCM). This flow rate is insufficient in a number of cases, for example for carrying out the treatment of effluent gases during the cleaning of thin-film deposition reactors for the fabrication of semiconductor devices.
Indeed, it is observed that an increase in the diameter of the tube for a given microwave power transmitted to the discharge tube, for a fixed flow rate, leads above a fairly low value to a decrease in the conversion yield. This can be explained, in particular, by the fact that an increase in the diameter of the discharge tube reduces the average energy of the electrons, which reduces the excitation by electron collisions.
Another important phenomenon connected with the diameter of the discharge tube is that of the radial contraction of discharges at atmospheric pressure, irrespective of whether they result from a DC, radio frequency or microwave electric field. This phenomenon becomes even more marked if the gas has a low thermal conductivity, as is the case with krypton.
If the contraction is significant, the plasma appears in filamentary form. As soon as there is contraction, the effect experienced by the gas as a result of the discharge becomes less and less as the distance from the axis of the tube increases.
In consequence, an increase in the diameter of the discharge tube is accompanied by the generation of one or more plasma filaments, between which the gas to be excited can flow without undergoing any excitation.
Furthermore, for a given gas flow rate, the power transmitted to the surface-wave discharge, to be increased with a view to increasing the conversion yield, cannot be increased, beyond a limit value, without risking the generation of an electric arc in the starting gap of the device.
The object of the invention is to overcome the drawbacks of the devices of surfaguide and surfatron-guide type in the prior art, and to provide a gas exciter device which makes it possible, on the one hand, to increase the gas flow rate for a given yield and, on the other hand, to increase the yield for a given gas flow rate.